If commodities X and Y are substitute, their cross elasticity of demand will be

If commodities X and Y are substitutes, the cross elasticity of demand between them will be positive.
Cross elasticity of demand measures how the quantity demanded of one commodity changes in response to a change in the price of another commodity. When two commodities are substitutes, they serve similar purposes and are considered as alternatives to each other. For example, if the price of commodity X increases, some consumers may switch to commodity Y as a substitute, causing an increase in the quantity demanded of commodity Y.
The positive cross elasticity of demand reflects this relationship. It means that an increase in the price of commodity X will lead to an increase in the demand for commodity Y, and vice versa. On the other hand, if the cross elasticity of demand were negative, it would indicate that the two commodities are complements, meaning that they are used together and a change in the price of one would result in an opposite change in the demand for the other.
Therefore, in the case of substitute commodities, the cross elasticity of demand will be positive.